Heat transfer occurs in three different modes; conduction, convection and radiation. Conduction arises from temperature gradients within a material wherein, in general terms, energy is transferred during collisions of adjacent molecules and the migration of free electrons from the warmer gradient areas to the cooler gradient areas.
Convection is heat transferred between a solid surface and an adjacent moving fluid. In accordance with Newton's law of cooling, q=hA.DELTA.T where "q" represents the heat transfer rate (BTU/hr), A the area of interface between solid and liquid and "h" the surface coefficient of heat transfer.
When two objects at different temperatures are placed a finite distance apart in a perfect vacuum, a net energy transfer occurs from the higher temperature object to the lower temperature object even though there is no medium between them to support heat transfer by either conduction or convection. This net energy transfer results from the third mode of heat transfer called thermal radiation, or simply, radiation. Any surface at an absolute temperature above zero degrees Rankine is found to continually emit energy carrying electromagnetic waves. The rate at which any given surface emits radiant energy per unit area of surface is a complex function of the surface temperature, type of material, and surface condition. However, for the class of surfaces defined as black bodies, which absorb all incident radiant energy, the emission rate is given by a simple expression called the Stefan-Boltzmann law which states that EQU W.sub.b =.delta.T.sup.4, (1)
where W.sub.b is the emission rate of a black surface per unit area of surface with units of BTU/hr ft.sup.2, T is the absolute temperature in degrees Rankine, and .delta. is a universal constant which is given by EQU .delta.=0.1714.times.10.sup.-8 BTU/hr ft.sup.2 .multidot.R.sup.4 (ii)
in a system of units consistent with the ones chosen for W.sub.b and T. Real surfaces emit at a rate lower than a black surface at the same temperature, although some, such as graphite and soot, come fairly close to the black surface emission rate given by (i) above. For an actual surface at temperature T with an emissive power W which is less than W.sub.b at temperature T, the expression for the emission rate is EQU W=.epsilon..delta.T.sup.4, (iii)
where .epsilon. is the total hemispheric emissivity and depends upon the type of material, surface temperature, and surface condition; .epsilon. is a number between zero and unity. Some values of .epsilon. for different materials subject to relatively low temperatures are provided below.
______________________________________ Silver, highly polished 0.02 Gold, polished 0.02 Platinum highly polished 0.05 Zinc, highly polished 0.05 Aluminum, highly polished 0.08 Monel metal, polished 0.09 Nickel, polished 0.12 Copper, polished 0.15 Stellite, polished 0.18 Cast iron, polished 0.25 Monel metal, oxidized 0.43 Aluminum paint 0.55 Brass, polished 0.60 Oxidized copper 0.60 Oxidized steel 0.70 Bronze paint 0.80 Black gloss paint 0.90 Concrete, rough 0.94 White lacquer 0.95 White vitreous enamel 0.95 Asbestos paper 0.95 Green paint 0.95 Gray paint 0.95 Lamp black 0.95 Paints, oil, all colors 0.89 to 0.97 ______________________________________
The distribution of thermal radiation in wavelengths is primarily between 0.1 and 100 microns which includes the visible radiation between about 0.3 and 0.7 microns. Infrared radiation is generally held to cover the range of wavelengths extending from 0.8 microns to 1000 microns.
The human body, due to its maintenance of a constant internal temperature emits copious amounts of infrared radiation primarily in the thermal wavelength bands of 3-5 microns and 8-14 microns. Various structures such as buildings, ships, tanks, etc. also give off copious amounts of radiation. The radiation emitted by persons and various structures is easily detectable by current infrared sensors used by military services for surveillance and gunnery.
Those objects emitting the higher amounts of radiation appear as the brightest objects to the infrared detection devices while the objects giving off only a small amount of radiation appear essentially as a black body. The objects which will emit the highest amounts of radiation are those having the highest emissivity values, highest temperature or both as made evident by equation (ii) above. Those objects giving off the least amount of radiation are those objects having low emissivity values and/or low temperatures. The objects emitting radiation in amounts between these extremes appear as various shades of gray to the detection device.
In many instances, it is desirable that personnel or equipment be able to remain undetected when facing these sensors.